312 335 Ch18 RE 896315.qxd 3/26/10 3:44 AM Page 318 S ... two nitrogen atoms combine, they share three electrons with each other. The bond between the two atoms has six electrons,

chapter

18

Read to LearnGain or Loss of Electrons

Imagine that you and a friend decide to go to the movies.You find out that you do not have enough money to buy aticket. But your friend has enough money for both of you andloans you the money. Now you both have enough money toget tickets to go to the movies.

Just as a friend can loan you money, atoms can loan electrons to other atoms. Atoms loan electrons so both canreach a stable energy level. When atoms gain, lose, or shareelectrons, an attraction forms between the atoms. The atomsare pulled together to form a compound. The attractioncreates a bond that joins the atoms.

A compound is completely different from the atoms thatmake it up. A compound has different physical and chemicalproperties from the atoms that are in it. A compound is twoor more atoms bonded together.

What are some common compounds?Ions form when atoms gain or lose electrons and become

changed particles with more or less electrons than protons.Many common compounds are formed by the loss or gain ofjust one electron. These compounds are made from a Group1 element, such as sodium, and a Group 17 element, such aschlorine. Examples are sodium chloride, also known as tablesalt, and potassium iodide, an ingredient in iodized salt.

318 CHAPTER 18 Chemical Bonds

Some atoms share electrons and become more stable.Describe a situation in which people share something andeveryone benefits.

What You’ll Learn

■ what ionic bonds andcovalent bonds are

■ what particles are produced by ionic andcovalent bonding

■ about polar and nonpolar covalentbonds

Before You Read

1. Explain What is a compound?

Locate InformationUnderline every heading in thesection that asks a question.Then, highlight the answers tothose questions as you findthem.

Chemical Bonds

section ● Types of Bonds2

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312_335_Ch18_RE_896315.qxd 3/26/10 3:44 AM Page 318 S-034 113:GO00492:GPS_Reading_Essentials_SE%0:XXXXXXXXXXXXX_SE:Application_Files_

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Reading Essentials 319

How is potassium iodide formed?What happens when potassium and iodine bond to form

potassium iodide? The figure above shows the electron dotdiagrams and electron distribution for this process.

An atom of potassium has one electron in its outer level.This is not a stable outer energy level. When a potassium atomcombines with an iodine atom, the potassium atom loses theone electron in its outer level. The third energy level nowbecomes a complete outer level. The potassium atom now hasone less electron than it has protons. The positive and negativecharges are no longer equal. Recall than an ion is a chargedparticle that is formed when an atom gains or loses electrons.The potassium atom is now a positive ion. It has more protonswith positive charges than electrons with negative charges.

The potassium ion has a 1� charge. The symbol for a positive potassium ion is K�. The plus sign shows its positivecharge. In the symbol K�, the plus sign is a superscript.Superscript means “written above.”

How does the iodine atom change?The iodine atom also changes when it combines to form

potassium iodide. An iodine atom has seven electrons in itsouter energy level. A stable outer energy level has eight electrons. When the iodine atom reacts with the potassiumatom, the iodine atom gains one electron from potassium.There are now eight electrons in iodine’s outer energy level.

The iodine atom now has a stable outer energy level, but ithas one more electron than it has protons. It has more negative particles than positive particles. The iodine atom hasbecome a negative ion with a charge of 1�. Now it is calledan iodide ion and its symbol is written as I�.

IK IK→

→K I� �

K IPotassium Iodine Potassium iodide

2. Describe How many electrons are there inpotassium’s outer energylevel after the bond hasformed?

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Ionic CompoundsAn ionic compound forms when two or more ions

combine. Notice that the ionic compound potassium iodidedoes not have a charge. The 1� charge and the 1� chargecancel each other out. This means the compound is neutral or has a neutral charge.

The Ionic BondAtoms that lack electrons in their outer energy level seem

to send out messages to attract atoms that will complete theirouter energy levels, and vice versa. A bond forms when ionsattract each other and form a compound. An ionic bond is theforce of attraction between a positive ion and a negative ion inan ionic compound.

In an ionic compound, one atom gives up electrons and the other atom takes them. Electrons are not shared in ioniccompounds. When an atom loses electrons, one or moreatoms must gain the same number of electrons that were lost.That way, the compound stays neutral. When atoms form anionic compound, a large amount of energy is released. Thisoccurs because of the transfer of electrons.

What if many electrons are transferred?Potassium iodide forms an ionic bond when one electron is

transferred. What happens when more than one electron istransferred? The figure below shows the formation of anotherionic compound, magnesium chloride, MgCl2.

When magnesium reacts with chlorine, a magnesium atomloses the two electrons in its outer energy level. The atombecomes a positively charged magnesium ion. The symbol forthis ion is Mg2� because it has lost two electrons and now hasa charge of 2�.

Two chlorine atoms each take one of the electrons andcomplete their outer levels. Each chlorine atom becomes anion with a 1� charge. Cl– is the symbol for the chloride ion.

In this case, the magnesium atom has two electrons to give.However, a chlorine atom can only use one electron. So, it takestwo chlorine atoms to receive the two electrons from themagnesium atom and combine to form the ionic compoundmagnesium chloride.

Magnesium chloride

Mg2� �Cl�Cl

Magnesium 2 chlorine atoms

ClMg

Cl→

3. Think Critically Explain whymagnesium chloride cannot be made from oneatom of magnesium andone atom of chlorine.

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●B Build VocabularyMake a Foldable like the onebelow. As you read this section,add the definitions of ionicbonds, covalent bonds, polarmolecules, and nonpolar mole-cules to your Foldable.

IonicBonds

CovalentBonds

PolarMolecules

NonpolarMolecules


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Does an ionic compound have a charge?The ionic compound magnesium chloride is neutral. It

does not have a charge. The compound is neutral because thesum of the charges on the ions is zero. The 2� charge on themagnesium ion is equal to two 1� charges on the chlorideions. When different atoms form an ionic compound, theytransfer their electrons. The total number of electrons andprotons stays the same. The compound is neutral.

Metals and nonmetals usually combine by forming ionic compounds. Elements on the far left side of the periodic tabletend to form ionic bonds with elements on the far right. Ioniccompounds are often crystalline solids with high melting points.

Sharing ElectronsSome atoms of nonmetal elements become more stable when

they share electrons. Look at the elements in Group 14 of theperiodic table. These elements have four electrons in their outerlevels. They would have to gain or lose four electrons to have astable outer energy level. It takes a lot of energy for an atom tolose or gain that many electrons. Each time an ion loses an electron, the nucleus holds the remaining electrons even moretightly. Once one electron is removed, it takes more energy toremove a second electron. It takes even more energy to removea third, and so on. For elements in Group 14, it is much easierfor them to become stable by sharing electrons.

Atoms that share electrons form a covalent bond. A covalent bond is the force of attraction between two atomsthat share electrons. A molecule is the neutral particle thatforms when atoms share electrons.

How are single covalent bonds formed?A single covalent bond forms when two atoms share two

electrons. Usually one electron comes from each atom in thecovalent bond. Look at the water molecule in the figure. Thereare two single covalent bonds in a water molecule. In each singlebond, a hydrogen atom and an oxygen atom each give one electron, which the atoms share.

Recall that ionic bonds help atoms become more stable byfilling their outer energy levels. Covalent bonds also help atomsfill their outer energy levels and become more stable.

OH

H


5. Locate Circle the electrons that could havebelonged only to thehydrogen atoms beforethey bonded with the oxygen atom.

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4. Explain Why is the ioniccompound magnesiumchloride neutral?

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What are multiple bonds?A covalent bond can have more than two shared electrons.

Look at the dot diagram of the two nitrogen atoms. Eachnitrogen atom has five electrons in its outer energy level. Anitrogen atom needs to gain three electrons to have a stablenumber of eight electrons in its outer energy level.

When two nitrogen atoms combine, they share three electrons with each other. The bond between the two atomshas six electrons, or three pairs of electrons. Each electronpair is a covalent bond. Three pairs of electrons form a triplebond. By sharing the electrons, each nitrogen atom now haseight electrons in its outer energy level. The symbol for the nitrogen molecule is N2.

Can double bonds occur?Molecules also can have double bonds. A double bond is

two pairs of electrons shared between two atoms. In the carbon dioxide molecule, CO2, the carbon atom shares twoelectrons with one oxygen atom and two electrons with theother oxygen atom. In return, each oxygen atom shares twoelectrons with the carbon atom. In this way, all three atomshave eight electrons in their outer energy levels. This makesall three atoms stable.

When electrons are shared in covalent bonds by similar oridentical atoms, such as in N2 or O2 molecules, the electroncharge is shared equally across the bond. A nonpolar bond isa covalent bond in which electrons are shared equally by bothatoms. Nonmetals commonly form nonpolar bonds.

Are electrons always shared equally?Atoms in a covalent bond do not always share electrons

equally. The positive charge of an atom’s nucleus attracts theelectrons in a bond. Some nuclei attract electrons morestrongly than others. If a shared electron is closer to onenucleus in a bond, that nucleus could attract the electronmore strongly.

You can see this with a magnet and a piece of metal. Whenthe magnet is closer to the metal, it attracts the metal morestrongly. Some nuclei have a greater positive charge than others. Nuclei with a greater positive charge attract electronsmore strongly. In the same way, a strong magnet holds themetal more firmly than a weak magnet.

� →N N NN6. Identify Highlight theelectrons that are beingshared in the diagram ofthe nitrogen molecule.Circle the electrons that arenot being shared.

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How can you picture the sharing of electrons?Imagine a game of tug-of-war. A covalent bond is like the

rope in a game of tug-of-war. The shared electrons are like the knot in the center of the rope. Look at the diagram of amolecule of HCl (hydrochloric acid) below. Each atom in themolecule of HCl is attracting the shared electrons. However,the atoms are not the same size. Then what happens?

The chlorine atom pulls on the electrons more than thehydrogen atom. The chlorine atom is like the stronger team intug-of-war. When one team is stronger, the knot in the middleof the rope moves towards the stronger team. A similar thinghappens in the molecule. The shared electrons are held moreclosely to the atom with the stronger pull.

What is an example of unequal electron sharing?

The covalent bond in a molecule of hydrogen chloride,HCL, is an example of unequal electron sharing. When HClmixes with water, it becomes hydrochloric acid. Hydrochloricacid is used to clean metal in factories. It is also found in yourstomach where it helps digest food.

Because of its size, the chlorine atom attracts the electronsin the bond more strongly than the hydrogen atom. Thatmeans the electrons in the bond spend more time closer tothe chlorine atom than to the hydrogen atom. The chlorineatom has a partial negative charge when the bonding elec-trons are closer to it. The hydrogen atom then has a slightpositive charge when the electrons are farther away from it.The bond is said to be polar. A polar bond is a covalentbond in which the electrons are unequally shared, producinga molecule with a negatively-charged end and a positively-charged end.

8. Apply Why does the chlorine atom have a partialnegative charge in a molecule of hydrogen chloride?

7. Label Draw an arrowshowing the pull ofelectrons toward thechlorine atom.

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H

(partial positive charge)

(partial negativecharge)

��

��

Cl


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What are polar and nonpolar molecules?The figure shows the energy levels of a water molecule. The

oxygen atom forms a covalent bond with each hydrogenatom. The oxygen atom has a stronger attraction for thesebonding electrons. Two electron pairs spend more time closerto the oxygen atom than to the hydrogen atom. This gives theoxygen atom a partial negative charge.

Because it has an end that is partially positive and an endthat is partially negative, water is a polar molecule. Polarmeans “having opposite ends.” A polar molecule is a moleculethat has a slightly positive end and a slightly negative end, butthe molecule itself is neutral.

When two atoms that are exactly alike form a covalentbond, they share electrons equally. In a nonpolar molecule,the electrons are shared equally in the bond. A nonpolarmolecule does not have oppositely charged ends. Atoms of thesame element can form nonpolar molecules.

Properties of CompoundsYou have just learned about ionic bonds and covalent

bonds. A covalent compound is one whose atoms are heldtogether by covalent bonds. Sugar is a covalent compound.An ionic compound is one that is held together by ionicbonds. Table salt, NaCl, is an ionic compound.

From the way they look, you might think that sugar andsalt are similar compounds. They are both little white grains,and they are both used in foods. They look like they mighthave the same physical and chemical properties.

Both sugar and salt are used to change the taste of foods.You add sugar to lemonade or tea to make them taste sweet.Table salt is used to enhance the taste of foods such as meatand vegetables. The reason sugar and salt have different effectson the taste of foods is because they have different physicaland chemical properties.

H2OWater

��

��

H H

O

10. Explain Although sugar andsalt look alike, why do theyhave different effects on thetaste of food?

9. Identify Highlight the partsof the water molecule thathave a slight positivecharge. Circle the part thathas a slight negative charge.

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How are covalent and ionic compounds different?

Ionic and covalent compounds have different physical andchemical properties. These differences are due to differencesin the attractive forces of the bonds.

What are some properties of covalent compounds?

Covalent bonds are strong, but the attraction betweenindividual molecules of a covalent compound is weak. Theseweak forces between individual molecules are responsible forthe properties of covalent compounds.

For example, melting and boiling points of covalent compounds are low, compared to those of ionic compounds.Sugar will melt at 185° C, but table salt will melt at 801° C.Covalent compounds form soft solids. They are poorelectrical and thermal conductors. They do not conduct heatand electricity well.

What are some properties of ionic compounds?The ionic bonds between ions are relatively strong. This is

why ionic compounds like table salt have high meltingpoints. They also have relatively high boiling points. Ioniccompounds are hard and brittle solids. If ionic compoundsare dissolved in water, they become good conductors ofelectric current.

Strong attractive forces hold the ions in place in an ioniccompound. Because of this strong attraction, ionic compounds are stable. When ions are drawn together, energy is released. The table below shows some of the different properties of covalent and ionic compounds.

Table 2 Comparison of Covalent and Ionic Compounds

Covalent Compounds Ionic Compounds

Bond Type Electron Sharing Electron Transfer

Melting and Boiling Lower Higher Points

Electrical Poor Good Conductivity

State at Room Solid, liquid, or Solid Temperature gas

Forces Between Strong bonds between Strong attraction Particles atoms; weak attraction between positive and between molecules negative ions

11. Explain Describe thestrength of the forcesbetween covalentmolecules.

12. Identify Highlight theinformation in the tablethat shows that sugar meltsat a lower temperaturethan salt.

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1. Review the terms and definitions in the Mini Glossary. Choose two terms that are relatedand write a sentence that uses those two terms.

2. Complete the outline to help you organize what you learned about chemical bonds.

3. You underlined every heading in the section that asks a question and thenhighlighted the answers. How did this help you learn the content of this section?

chemical bond: the force that holds atoms together in a

compound

covalent bond: the force of attraction between two atoms

that share electrons

ionic bond: the force of attraction between a positive ion and

a negative ion in an ionic compound

molecule: the neutral particle that forms when atoms

share electrons

nonpolar bond: a covalent bond where the electrons are

shared equally

nonpolar molecule: a molecule in which the electrons are

shared equally in a covalent bond

polar bond: a covalent bond where electrons are unequally

shared producing charged molecules

polar molecule: a molecule that has a slightly positive end

and a slightly negative end, but the molecule itself

is neutral

After You ReadMini Glossary

End of

Section

Types of Bonds

I. What is an ionic bond?

A. Forms between a positive and a negative ion

B. Electrons are

C. Usually forms between a metal and

D. Forms a(n) compound

II. What is a covalent bond?

A.

B. Electrons are

C.

D. Forms a

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312 335 Ch18 RE 896315.qxd 3/26/10 3:44 AM Page 318 S ... two nitrogen atoms combine, they share three electrons with each other. The bond between the two atoms has six electrons,